As known hitherto, a feed rod is forwarded from a direction nearly orthogonal to the axial line of an electrode, when a projection nut held in the feed rod comes just above a guide pin, the held projection nut is released and dropped into the guide pin. Such an example is disclosed in Japanese Laid-open Patent No. 51-109247. The disclosed art is as shown in FIG. 10, in which a hollow feed rod 1 is disposed so as to move back and forth nearly in the horizontal direction, and a box type part container 2 is formed at the leading end thereof Since the lower side of the part container 2 is open, a sliding plate 3 is placed, and the sliding motion is effected by a reciprocating rod 4. A steel plate part 6 is mounted on an electrode 5, and a guide pin 7 of the electrode 5 projects upward while penetrating through the steel plate part 6. The part is a projection nut indicated by reference numeral 8. The other electrode making a pair with the electrode 5 is omitted in the drawing.
FIG. 10 shows the projection nut 8 being supplied on the steel plate part 6 as indicated by solid line as the feed rod 1 moves forward, and the operation is described below. The feed rod 1 moves forward from a direction nearly orthogonal to the electrode 5, and when the nut 8 indicated by twin dot chain line comes to be coaxial with the guide pin 7, the feed rod 1 stops, and then the reciprocating rod 4 is moved back, and the lower side of the part container 2 is opened, so that the nut 8 drops, with the guide pin 7 driven into the screw hole as indicated by solid line, thereby finishing the feed of the nut. Then, the other electrode moves forward, and projection welding is completed.
In such prior art, since the projection nut 8 is dropped onto the guide pin 7, if the nut 8 is inclined in the transient period of fall, it may be repelled by the end of the guide pin 7, and may not be placed correctly as indicated by the solid line. Such an abnormal phenomenon may also occur if the feed rod 1 does not stop at a correct position due to a relative configuration with the guide pin 7. Moreover, the projection nut may be deviated in the rotating direction from the guide pin 7, and may not be welded in a uniform direction on the steel plate part 6.